The present invention relates to a screw-nut device comprising a screw and a nut with multiple threads of identical pitch, and rolling elements disposed between the threads of the screw and of the nut.
Screw-nut devices, whether they are of the type with circulation of balls or with satellite rollers, are used in order to convert rotation movements into translation movement and vice versa.
In a screw-nut device of the type with balls or of the type with satellite rollers, the rolling elements are in oblique contact with the sides of the helicoidal grooves or threads of the screw and of the nut, at an angle whose nominal value is generally close to 45.degree. so as to ensure at the same time the relative centring of the screw and of the nut and the taking up of the mainly axial and occasionally radial loads.
For an assembly with clearance, the usual design is such that all of the contacts of the rolling bearing elements (balls or rollers) with the sides of threads of the screw and of the nut are made at an angle having the same orientation, for a given direction of the external axial force transmitted by the rolling elements, so as to distribute the load over a maximum of contact points.
FIG. 1 of the accompanying drawings shows the contact conditions which are established between the balls B and the threads of the nut E and of the screw V in a screw-nut device with clearance, under the effect of an axial load C. It will be recognised that here all the balls B are in contact at an angle .alpha. of the same value and of the same orientation with the sides of all the threads.
In order to increase the precision of such a screw-nut device, it is usual to adjust or to reduce to zero the internal clearance of the device or to produce an internal preload.
For this, several systems are currently used.
1. Monobloc nut and rolling elements of larger diameter
The use of rolling elements (balls or satellite rollers) of larger diameter enables the internal clearance to be eliminated, the rolling elements then being in contact with both sides of the threads of the screw and of the nut.
In the case of screw-nut devices with balls, the threads generally have a profile constituted by two arcs of a circle so as to maintain angles of contact of the order of 45.degree., and each ball is then in contact with four sides, as is shown in FIG. 2 of the accompanying drawings. The axes of rotation of the balls on themselves thereby find themselves modified in comparison with the instantaneous axes of rotation in the case of two directly opposed contacts, and this results in increased sliding phenomena, creating an increase in the friction torque. The necessary torques are then higher and more sensitive to variations in the diameter of the screw, and the life of the assembly is reduced because of internal frictions contributing to wear.
For screw-nut devices with satellite rollers, the axis of rotation of the rollers does not change, whether the assembly is preloaded or not, but the construction of rollers to the precise diameter required in order to obtain a given preload is technically and economically more difficult because rollers constitute more complex elements than balls.
2. Reduction of the pitch circle diameter of the nut, by deformation of the latter
Instead of interposing the rolling elements of greater diameter, one may proceed to a reduction of the pitch circle diameter of the nut by deformation of the nut. The phenomenon is then the same as far as the modification of the axis of rotation of the balls is concerned, with in addition, for certain embodiments, the disadvantage that the track for rolling in the nut is interrupted by a slot, in order to facilitate the deformation of the nut. The points of contact with the rolling elements on either side of this discontinuity have of course the disadvantage of causing a concentration of load accelerating the phenomenon of fatigue at these points.
3. Nut formed of two sections adjustable with respect to one another
Various known systems exist using nuts formed of two sections with the capability of positioning or of adjustment of two sections with respect to one another, so that the rolling elements (balls or rollers) are in contact with different sides of the threads when one of the two sections is compared with the other.
a) By relative translation between the two sections of nut.
By adjustment of the axial distance between the two sections of nut, the latter maintaining the same relative angular position, it is possible to adjust the clearance or to create a preload.
This relative translation may be produced, for example
by adjustment of the two opposite surfaces of the two sections, bearing directly against one another or onto a rigid or resilient spacer washer, PA1 by action on the outer surfaces of the two sections of nut enclosed in a housing, PA1 or by any other system of the same principle, the two sections of nut maintaining the same relative angular position during the adjustment.
b) By relative rotation between the two sections of nut.
The two sections of nut being supported axially in a fixed and opposed manner with respect to reference surfaces (outer or inner surfaces supported directly or with intermediate spacers), any angular phase displacement by rotation of one of the two sections with respect to the other amounts to bringing together or separating the opposite sides of the threads of the two sections and thus causing a result identical to that of the translation described under a).
There exist various known systems for obtaining this relative angular displacement, in a definitive or an adjustable manner, and for maintaining the relative angular position of the two sections once the adjustment has been made.
According to the two above-mentioned methods of adjustment, by translation or relative rotation of the two sections of nut, the final result is seen as a difference between the nominal pitch and the actual distance between the threads in the zone of transition between the two active parts of the threads borne by the two sections.
FIGS. 3a and 3b of the accompanying drawings show the conditions of contact of the balls B with the threads of the screw V and of the two sections of nut E.sub.1 and E.sub.2 in two different relative positions of these two sections of nut E.sub.1 and E.sub.2, namely in FIG. 3a with a distance D-.DELTA. between the two sections of nut E.sub.1, E.sub.2 and in FIG. 3b with a distance d+.DELTA. between the said sections of nut.
4. Monobloc nut with adiustment of the clearance by central phase displacement of the threads of the nut during the machining
Such a design of the nut makes it possible to dispense with the additional machining operations necessary in order to produce the centring and the adjustment of positioning of the two sections of nut when these two sections are executed in two pieces, as indicated under 3), and therefore permits better concentricity while being more economical for single-thread assemblies with balls. It also retains, for single-thread assemblies with balls, the advantage of an oblique contact limited to two opposed points per ball, thus avoiding the disadvantages of the systems with four contact points per ball (friction, wear).
FIGS. 4a and 4b of the accompanying drawings show such a device comprising, associated with a single-thread screw V of pitch p, either a monobloc nut E.sub.a having a thread of pitch p, except in a central position where the gap between two consecutive threads is reduced to p-.DELTA., as shown in FIG. 4a, or a monobloc nut E.sub.b on which, in a central position, the gap between two consecutive threads of pitch p is increased to p+.DELTA., as shown in FIG. 4b.
Both for the devices having two separate sections of nut in accordance with 3) and for devices having two monobloc sections of nut in accordance with 4), the nut must comprise an independent circuit of balls for each of the two sections of nut, so as to avoid on the one hand a too large number of balls per circuit and on the other hand the irregularities of internal friction which would be produced at the passage of the balls, in the transition zone between the two sections, from a given angle of contact to an angle of contact of opposite direction.
Moreover, when the advance per revolution (that is to say the pitch) is large, the total length of the nut is then increased insofar as the length of each section of nut must be at least equal to the pitch (equivalent to one turn of balls). This involves technological problems for the production of monobloc nuts where rectified nuts are concerned, and becomes impossible to produce if one wishes to consider a more economical machining by tapping.
In conclusion, known screw-nut devices with balls or rollers as currently used have, when means are provided for adjustment of the conditions of contact, particularly in the case of multiple threads, disadvantages either of a functional nature or of an economical nature.